Skin cancer is the most prevalent form of cancer today. This can be attributed to the damaging effects of sunlight which causes the production of DNA photoproducts, otherwise known as photolesions. Researchers have been unable to derive conclusive structure activity relationships in light-induced mutagenesis due to the lack of pure well-characterized photolesion containing DNA for study. The object of this proposal is to determine the structure-activity relationships of the cis-syn, trans-syn, (6,4) and the newly discovered Dewar photoproducts of both TpT and TpC sequences. In addition we will examine the structure activity relationships of multiple lesion containing DNA. In order to do this we will construct site-specific photolesion containing dinucleotides, oligonucleotides, template-primers, DNA fragments, polymers and viruses for the determination of: (1) the structure of the further photoproduct of the (6-4) photoproduct formed in DNA. What is the role of this lesion in type III photoreactivation? (2) the photochemical, chemical and structural properties of photolesions. What role do lesions play in DNA-protein crosslinking and lesion hotspot formation. (3) the high resolution X-ray and combined 2-D NMR and molecular modeling structures of lesion containing duplexes. What role does structure play in recognition of lesions? (4) the mutation spectrum, of a lesion which results from polymerase bypass of a lesion or cluster of lesions. What contributes to high mutagenicity? (5) the relationship between the action of repair enzymes on lesion containing duplex DNA fragments and the amount of photolesion- induced DNA bending and other types of distortion. How are lesions detected? Are some lesions refractory to repair? (6) the lethality and mutation spectrum of lesions in vivo. How does this compare to the in vitro results? Hopefully this work will lead to a more complete understanding of the mechanisms of mutagenesis which will then enable us to better protect ourselves from the environmental mutagens and carcinogens to which we are constantly exposed.